Dual TDC Detector Array with Autocalibration for TOF-PET
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Solution Overview
Problem
Conventional radiation detectors for time-of-flight positron emission tomography (TOF-PET) face challenges in achieving sub-nanosecond temporal resolution due to coarse system clocks and spatial variations, leading to errors in temporal resolution and localization of positron emission events.
Innovation Solution
The implementation of a dual time-to-digital converter (TDC) system with automatic self-calibration and tunable delay elements in the trigger circuitry to correct for temporal drift and spatial skew, enhancing the precision of time stamping and spatial localization of radiation events.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a coarse system clock (e.g., 200 MHz) is used for time stamping, then the device complexity is reduced, but the temporal resolution is insufficient for sub-nanosecond TOF PET imaging
Solution Approach 1:
The time measurement function is segmented into two independent TDC channels, each with its own fine counter and clock reference. This segmentation allows each channel to operate independently with optimized timing parameters, achieving sub-nanosecond resolution without requiring an overly complex single-clock system
Solution Approach 2:
A delay element is introduced as an intermediary component between the trigger signal and the TDC input. This delay element provides precise temporal offset adjustment, enabling accurate time stamping relative to the system clock while maintaining sub-nanosecond resolution
2Device complexity
If trigger circuitry triggers on the first detected photon or a fixed photon count, then the device complexity is reduced, but spatial skew across the detector array causes substantial temporal errors
Solution Approach 1:
The trigger criterion is changed from a fixed photon count to a dynamic criterion based on accumulated photon count compared against a threshold. This parameter change allows adaptation to spatial variations in detector response, compensating for skew across the detector array while maintaining relatively simple trigger circuitry
3Device complexity
If voltage and temperature variations are not compensated, then the device complexity is reduced, but temporal drift in the fine counter output degrades effective temporal resolution
Solution Approach 1:
A feedback mechanism is implemented where the accumulated photon count is continuously monitored and used to adjust the trigger threshold. This feedback loop compensates for temporal drift caused by voltage and temperature variations, maintaining stable temporal resolution without requiring complex external compensation circuitry
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach improves the temporal and spatial resolution of radiation detection events, providing more accurate time-of-flight PET imaging by compensating for system clock limitations and spatial variations, thereby enhancing the overall performance of the PET detector array.
Implementation Method 1
a single 511 keV gamma ray produces a scintillation comprising many photons in the optical or other wavelength range
Implementation Method 2
silicon-based single-photon avalanche diode (SPAD) detectors... detect the 511 keV gamma rays in conjunction with a scintillator
Data Source
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AI summary
A detector (22) detects an event. First and second time-to-digital converters(TDCs) (70, 72) generate first and second time stamps (TS1, TS2) for the detection of the event. The first TDC and the second TDC are both synchronized with a common clock signal (62) that defines a fixed time offset between the second TDC and the first TDC. An autocalibration circuit (120) adjusts the first TDC and the second TDC to keep the time difference between the second time stamp and the first time stamp equal to the fixed time offset between the second TDC and the first TDC. The detector may be a detector array, and trigger circuitry (28) propagates a trigger signal from a triggering detector of the array of detectors to the first and second TDC's. Skew correction circuitry (132, 134, 136, 142, 60, 162) adjusts a timestamp (TS) based on which detector is the triggering detector.